In Situ Scanning Electron Microscopy High Temperature Deformation Experiments to Study Ductility Dip Cracking of Ni–Cr–Fe Alloys

An in situ high temperature-straining test associated to scanning electron microscopy has been implemented to study at the submicron scale different phase transformation and failure mechanisms phenomena in structural materials. This setup has been used to study the solid state cracking phenomenon known as Ductility Dip Cracking (DDC), which plagues some fcc metallic materials when strained at high temperatures. The Ni-base alloys AWS A5.14 ERNiCrFe-7 and ERNiCr-3 behavior were evaluated in situ at temperatures ranging from 700 to 1000°C.

The DDC susceptibility for both alloys was quantified using the threshold strain for cracking initiation (ε_min). The in situ results obtained at the sub-micron scale were compared with strain-to-fracture test results available in the literature, which are obtained at the macro scale using a thermo-mechanical simulator Gleeble^®. The ε_min measured by the in situ test for the ERNiCrFe-7 and ERNiCr-3 alloys was 7.5 and 16.5%, respectively, confirming the better resistance of ERNiCr-3 to DDC. In addition to the quantitative DDC susceptibility information, and most important, the in situ approach made possible the real-time observation of such failure phenomenon at the sub-micron scale. The grain boundary sliding associated to DDC was verified and quantified. Two differentiated components of grain boundary sliding: pure sliding (Sp) and deformation sliding (Sd) were quantified. Thus, a direct and quantitative link between grain boundary morphology (tortuosity), grain boundary sliding, and DDC resistance has been established for the ERNiCrFe-7 and ERNiCr-3 alloys.